Bug 1701276 - Adjust currentSrc behaviour to match the spec more closely. r=bryce

Firefox and Chrome support setting `src` with an `ObjectURL`. This is
technically a URL and should be treated as such. Firefox recently changed to
return `""` when a `MediaSource` is playing, and this is wrong, because even
though it's playing a from a "Media Provider Object", it got loaded from the
`src` attribute, so should have a non-null `currentSrc`, which should be the
blob url (`blob:origin/<an uuid>`). In this case, `currentSrc` is a string that
is opaque and doesn't allow introspection but refers to the "real" `MediaSource`
or `Blob`.

If the same `MediaSource` is loaded via the `srcObject` attribute (that Firefox
and Chrome don't support), then `currentSrc` should return `""` and `srcObject`
still has the object set to it to understand what the HTMLMediaElement is
playing. We'll implement and test this later.

Differential Revision: https://phabricator.services.mozilla.com/D110290

dom/html/HTMLMediaElement.cpp

@@ -6603,9 +6603,17 @@ void HTMLMediaElement::FireTimeUpdate(TimeupdateType aType) {
 MediaError* HTMLMediaElement::GetError() const { return mErrorSink->mError; }
 
 void HTMLMediaElement::GetCurrentSpec(nsCString& aString) {
+  // If playing a regular URL, an ObjectURL of a Blob/File, return that.
   if (mLoadingSrc) {
     mLoadingSrc->GetSpec(aString);
+  } else if (mSrcMediaSource) {
+    // If playing an ObjectURL, and it's a MediaSource, return the value of the
+    // `src` attribute.
+    nsAutoString src;
+    GetSrc(src);
+    CopyUTF16toUTF8(src, aString);
   } else {
+    // Playing e.g. a MediaStream via an object URL - return an empty string
     aString.Truncate();
   }
 }